The present invention relates to a heat sensitive color developing material which can be used as a recording material, and for other purposes.
A conventional heat sensitive color developing material is produced by dispersing an electron donating color forming organic compound and a phenolic compound as a color developer into a binder in high concentrations, and then coating it onto paper. As soon as the phenolic compound is fused by heating, a phenolic hydroxyl group allows the electron donating compound to develop color.
However, there are the following disadvantages in the conventional systems:
(1) In general, the phenolic compound is readily soluble in an organic solvent. When it is mixed with the electron donating organic compound, they both dissolve and color begins to form immediately prior to heating. Therefore, the materials do not have an opportunity to dissolve and mix uniformly with each other.
(2) It is difficult to disperse both materials, i.e., the electron donating organic compound and the phenolic compound into the binder in finely divided form (less than one micron) and uniformly.
(3) In order to maintain a suitable heat sensivity under a given temperature, it is adjusted by the melting point of the phenolic compound. Suitable phenolic compounds are limited so that it is difficult to achieve.
Japanese Patent Application Publication No. 62-263525 describes a heat sensitive color developing material using a compound or resin containing a neutral t-butyl ester as the electron donating compound, for example, a leuco compound. When the compound or resin containing the neutral t-butyl ester radical is heated at temperatures of 200.degree. C. or higher, it decomposes rapidly into carboxylic acid and isobutane. Due to the presence of the carboxylic acid, the leuco begins to develop color. This results in an improvement in the conventional heat sensitive color developing material using the phenolic compound as a color developing agent.
Japanese Patent Application Publication No. 63-325637 provided improvements in the above discussed material and found a color developer resolvable at lower temperatures and obtained a heat sensitive color developing material color developable at a lower temperature.